Modelling field hydrology in acid sulfate soil in Nordic conditions with different water management practices

Acid sulfate soils are the nastiest soil type in the world and the soils are located mainly in coast of the Gulf of Bothnia in western Finland. The large share of cultivated fields with acidic sulfate soils are drained because of waterlogging. Increasing of ground-water depth (GWD) by draining fields promotes oxidation of sulfidic materials, and the leaching of acidic water to surface water. Acid discharge from acid sulfate soils has been reported to deteriorate aquatic environment of surface waters. To prevent of acidic discharge formation and maintain GWD, controlled drainage (CD) is implemented. The aim of this thesis was to understand the effect of different water management practices (normal subsurface drainage, CD and CD with sub-irrigation) on GWD and subsurface drain discharge using theoretical description by way of modelling.

During this study, field hydrology of the silty clay loam agricultural field in Söderfjärden, western Finland, with Sulfic Cryaquepts soil type was modelled using two one-dimensional models (Drainmod-based model and HAPSU). The model input data were hourly hydrological data from Oct 2010–Dec 2017. The simulation accuracy was assessed graphically as well as by Nash–Sutcliffe model efficiency coefficient (NSE), mean absolute error (MAE) and bias. The models were able to simulate the long-term field hydrological processes with different water management practices. The performance of models for the normal subsurface drainage field was good with NSE value (0.62-0.68) compared with the CD field with NSE value (0.38-0.47). Overall, Drain-mod-based model had more satisfactory performance for CD field than HAPSU model.

Comparing the simulated and measured effect of water management practice revealed that not all water flow processes in the field were taken into account in the modelling. The monthly water balance analysis indicated that due to seepage GWD reached the layers below the drainpipe in winter, and deeper GWD was observed after dredging of the main ditch. The simulated drain discharge was clearly affected by water management practices but not seen from the long-term measurements. The models would be good tools for analyzing effects of different water management practices on water quality but the hydrology in the models is needed to be improved and flexibility to implement field operations like dredging and control structure can improve the performance of the models.